1. Field of the Invention
The invention relates to a circuit arrangement provided with a controlled switch which is controlled by means of a periodic switching signal for operating an oscillation circuit at a frequency f, and with a control circuit for generating the periodic switching signal, which control circuit comprises a pulse width generator for generating a periodic square wave signal with a half cycle duration which is adjustable in steps of a value T and which has a value of at least t.
2. Description of the Related Art
A circuit arrangement of the kind mentioned in the opening paragraph is known from EP-A-0 708 579. The known circuit arrangement is designed for igniting and operating a discharge lamp. The periodic switching signal traverses a number of discrete frequencies during a test phase for creating corresponding AC voltage frequencies at the output of an oscillation circuit comprising a resonant circuit including a self-inductance L and a capacitor C. The periodic switching signal is frequency-modulated during lamp operation with one or several of the discrete frequencies. The known circuit arrangement comprises a microprocessor of type ST6265, made by SGS Thomson, for generating the periodic switching signal. The microprocessor is provided with a pulse width generator consisting of a separate timer and a pulse width modulator (PWM) with which the desired discrete frequencies are realized in that the minimum half cycle value t and the step width T are laid down. The values of t and/or T are to be changed for obtaining other discrete frequencies. It is true that this provision makes it possible to realize a very large number of discrete frequencies over a wide frequency range and with a wide variation in step widths, but microprocessors fitted with such an option are comparatively expensive. Another disadvantage of the known circuit arrangement is that the microprocessor has only a very limited RAM capacity at its disposal and is programmable in machine language only. Microprocessors programmable in the programming language C are available with a very large RAM space. However, comparatively inexpensive embodiments of such microprocessors often do not have a separate timer and pulse width modulator (PWM) as the pulse width generator.